The purpose of this study is to define osteoclast specializations in bone resorption. We have demonstrated that in medullary bone of birds, osteoclasts are functionally synchronized depending on the need for calcium imposed by egg shell formation. This is the only system yet described where there are a large population of osteoclasts that are functionally and predictably synchronized and this offers a unique opportunity to study mechanisms of cell mediated bone resorption. It would be of interest to define the factor or factors that regulate calcium metabolism and bone cell changes during cyclic fluctuations in calcium requirements. Recent emphasis has been placed on the mechanisms of osteoclast activation either normally when calcium is needed for egg shell formation or when parathyroid hormone is administered. In both cases the osteoclasts are rapidly activated and this process is being characterized morphologically and histochemically.